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Chapter 5 OPPORTUNITIES IN PARTICULAR TECHNOLOGIES SUMMARY This chapter describes opportunities for research and development where advances in electrochemical devices and processes will probably have a significant economic impact in the near term (less than 10 years). Both new and traditional industries are considered. The current status and needs for research and technology development, along with some institutional issues, are examined for Batteries and fuel cells: Technical requirements are documented for advanced applications in ground-based vehicles, space and central electric utility systems, communication systems, medical applications, and weapons; associated research and development topics are summarized. · Biomedical science and health care: Electrochemical processes characteristic of living systems are reviewed, including such aspects as applications based on neuroscience, enzyme biocatalysis, adhesion and cell fusion, and electrophoresis. · Coatings and films: Most paints and coatings degrade by a photoelectrochemical mechanism. Applications are summarized that include protective coatings for automobiles, encapsulants for microelectronic devices, electrocatalysts, and microencapsulation techniques for controlled release of electroactive components. · Electrochemical corrosion: A framework of opportunities is presented with respect to corrosion research and engineering, dissemination of information, and new control technology to reduce corrosion losses. . · Electrochemical surface processing: Research and development underlying new monolithic and composite materials, coatings, electroplating and etching, and microelectronic devices, among others, are highlighted. · Manufacturing and waste utilization: Current applications and emerging technologies are reviewed, and dominant economic considerations are noted for electrolytic processes, electro-organic synthesis, 41
42 coproduction of metals and anodic products, and specific applications such as vehicles, electric power, and waste utilization. Membranes: Directions are outlined to achieve greater membrane stability and molecular transport and in turn to permit wider use of energy-efficient and economically attractive membrane technology in biotechnology, health care, and chemical synthesis. · Microelectronics: Electrochemical phenomena are essential in the manufacture of electronic and photonic systems as well as responsible for the quality and reliability of such systems. Applications and research are outlined in areas that include manufacture of microcircuits, interconnecting networks, lightwave communication devices, parallel processors, content-addressable memories, and nerve-electronic interfaces. · Sensors: Key technical problems involve materials and fabrication methods for both gas-and liquid sensors; opportunities for utilizing advanced microelectronics and membrane technologies are suggested for applications in 'environmental, industrial, and clinical systems, including in vivo monitoring of drug delivery systems. Electrochemical science and engineering is moving extremely rapidly in areas of advanced energy conversion devices, microelectronics, and sensors. These technologies have significant market growth potential, and international competition is keen. Greater support from both federal and industrial sources would have a major impact in these areas. BATTERIES AND FUEL CELLS The current and emerging applications for batteries and fuel cells are numerous and highly varied (1-4~. These chemical sources of electrical energy are absolutely essential for life in today's world. A sampling of current applications includes portable electric power for a wide range of civilian, industrial, military, and aerospace applications such as flashlights, radios, tools, medical devices (heart pacemakers, drug delivery systems), weapons, communication equipment, alarms, signals, and satellite power in space. All of the world's telephones operate with batteries as standby power sources. Standby power, emergency power, and uninterruptable power are provided by batteries for high-priority systems such as hospitals computers, and military weapons installa ~ . lions. Motive power is provided by batteries for hundreds of thousands of specialty vehicles such as forklift trucks, personnel carriers, air- port utility vehicles, submarines, torpedoes, and drone aircraft. New battery systems are being developed with far greater specific power and specific energy than realized in conventional batteries (Figure 5-1~.
200 100 ~0 SPECIFIC 60 POWER (W/kg) 40 30 20 Current _~__ Projected LiAl/FeS . Ii`,Si/FoS2 Chevette (1.4 liter) ~'A Zn/ NiOOH · ~ ~ ~ ~ rVa/S _ ~`~- ~ ~/ Acceleration (Peak) ~ ------- `---r'y--------------- %~\ \~t -- - --^\---`\ ~\ 1 ~ ~ ~ 1 /` ~ ~ Pb/PbO2 \ ~ ~ ~ 1 1 . ... ~ . 10 . . t --i; Uman {AV~ ) , . .. .. . .. I I I I I l I I l! . 11 &., . , .. 1 1 1 1 .. 1 10 20 30 40 60 80100 200 300400 600 8001000 SPECIFIC ENERGY (W.h/kg) FIGURE 5-1 Specific energy versus specific power for several batteries under development, compared to the Pb-PbO2 battery. Note that high specific power and high specific energy are offered by some of the new batteries. Emerging applications for fuel cells and batteries are oriented toward higher performance and longer life. In the near term (within a decade), advanced electrochemical power sources will be available to act as the principal motive power source on a commercial basis for delivery vans, buses, and other fleet vehicles. In the far term (more than a decade away) these types of power sources will become available for higher-performance automobiles, rail vehicles, high-performance submarines, ships, and perhaps aircraft. Stationary energy storage applications include storage in electric utility networks (near-term availability), wind-powered electric systems (near-term), and solar-electric systems (far-term). Fuel cells (Figures 5-2 and 5-3) are strong candidates in the far term for high-efficiency (greater than 50 percent) commercial electric utility power generation and stand-alone power generation for shopping centers, hospitals, military installations, and industry, as well as for remote power generation in developing countries. Also, fuel-cell-powered vehicles of all types are a far-term possibility. At present there is an increased interest in ultrahigh-performance electrochemical systems for defense and space applications. Some of these systems could find application within the next decade. New high-performance miniature batteries are in demand for medical applications, including mobile heart-pump systems, drug-delivery
44 H2 H2O - 3- _ , ~ = =: ~ an-' - = == ~9 . -=W ---~ L ~ =-=: .. ~ _ ~ ~ ~ ~- Air I Electrolyte l H2 Anode 2 2 2 O2 Cathode H2O N2, H2O FIGURE 5-2 Schematic cross section of a hydrogen-oxygen fuel cell, the heart of fuel cell systems. Such systems may be a major power source for electric utilities and electric vehicles. systems, and electrically powered prosthetic devices of various types. A number of these medical applications will be fulfilled within the next several years. The performance capabilities required of batteries and fuel cells vary according to the type of application. Some sample requirements are given in Tables 5-1 and 5-2. There are a number of barriers to achieving the requirements for various battery and fuel-cell applications. In general, improvements are needed in the areas of initial cost, lifetime, and performance.
45 1orooo 3000 2000 u' a) U1 1000 ~ BOG . _ ~ 600 cn , 400 o a) 200 100 1 1 20 . ; ~40V \~\ 3.0 · Lit 2.0 \\\ ZniAir~Li/~\ Li4Si/FeS2t Li/Fe \\\ \/Ai e:Na/S~ <~Na/SbC13 Li4si/Fcse~LiAllFes2\ \ \ LiAI/F~ 6~ In/NiOOH \\\ \ \ \ Nags \ \ Cd/Ag2O2- ·Fe/NiOOH \ \ \ Zn/HgO \ \ \ \ \ \ ·Cd/NiOOH \ \ 1 1 1 - - ~\\~ 1 ~ 1 \1 ~ N~ 1360 100 Equivalent Weight' g/equiv 104: a) llJ C' ._ · _ Cal a) . En O 1o3 ~ 1 1 000 FIGURE 5-3 Theoretical specific energy for electrochemical cells. An opportunity exists for the development of systems that have the capability of storing 5 to 10 times more energy per unit weight than the Pb-PbO2 cell. More specific barriers to meeting the goals include the high cost of electrocatalysts and some porous electrodes; the prevention of corrosion of active and passive cell components; instabilities of porous electrode structures under long-term cycling; loss of electrocatalytic activity with time and use; susceptibility of electrolytes to oxidation and/or reduction by electrode reactants; inadequate conductivity of electrolytes for high-performance applications; inadequate membranes and separators (low chemical stability and conductivity); passivating film formation on electrodes; and lack of advanced electrode and cell designs for high-performance applications.
46 TABLE 5-1 Performance Requirements for Batteries in Advanced Applications Specific Specific Battery Energy Power Efficiency Lifetime Cost Application (Wh/kg) (W/kg) (%) Cycles (years) ($/k\\'h) Autos and vans >70 >120 >60 >300 >3 <100 Stationary energy storage n/a n/a >70 >2000 >10 <100 Portable power for electronics >250* >2 n/a primary various 4000 Weapons (example) >100 >200 -- various various - NOTE: Efficiency (%) = percentage of theoretical efficiency. *The additional volumetric requirement of >0.6 Wh/cm3 is very important. TABLE 5-2 Performance Requirements~for Fuel Cells in Advanced Applications Specific Fuel Cell Power Efficiency Startup Cost Application (W/kg) (%) Time Lifetime ($/kW) Autos and vans 120 >30 <20 see Stationary utility n/a >40 <1 hr Weapons (example) >1000 >3 years <75 >10 years <1000 < 1 min < 1 hour Space power >100 >40 <1 min >5000 hours - NOTE: Efficiency (%) = percentage of theoretical efficiency.
47 A somewhat more general consideration of the battery and fuel-cell field reveals a number of generic problems that are important in numerous other electrochemical systems: · The dimensional and morphological stability of porous electrode structures under operating conditions · Chemical and physical control of electrocrystallization of metals and their solid discharge products · Gas evolution at electrodes (H2 and/or O2 in aqueous systems) · Electrocatalysis of O2 reduction and evolution · Optimization of transport processes in porous electrode systems (gases, ions, electrons, solvents) · Electrocatalysis of the oxidation of logistic fuels (hydrocarbons' reformer gas, methanol, coal) Suppression of passive film formation ~ Advanced methods for the design and optimization of electrodes, cells, and electrochemical systems · Advanced methods for in situ study of electrochemical and chemical reactions in porous electrodes and immobilized electrolytes A plan for a more vigorous electrochemical R&D program (at a funding level at 2 to 3 times the present value) would, for research, enhance the funding and staff of existing programs of electrochemical research and focus added effort on the generic problems discussed here. For development, the plan would establish initiatives (described in Chapter 4) for each of the systems undergoing development berg., Na-S, Zn-Br2, Li-FeS2, H2-M2CO3-air, H2-ZrO2-air). BIOMEDICAL SCIENCE AND HEALTH CARE The origin of electric potentials in biological systems arises from the existence of free ions, ionized molecular groups, or electrically polarized biomolecules. In addition, electrical potentials accompany charge transfer processes during the reaction of biologically active systems. Although many processes that occur in biological systems lie outside the scope of this report, and although advances in these areas are likely to be made in a wide variety of disciplines, there are some key areas where electrochemical phenomena play a significant role. For example, the processes characteristic of living systems, such as active transport and secretory processes, photosynthesis, sensory and energy transduction, conduction and transmission of impulses, motility, and reproduction, are all based on interactions between ions, poly- electrolytes (proteins, DNA), or charged membranes containing enzymes and ion-selective channels. The units of these biological structures
48 are charged, and their interactions involve electrical forces. An understanding' of life processes may thus be greatly aided by collaboration with individuals who possess a thorough grounding in electrochemical concepts and techniques. Such knowledge is also indispensable for developing ways of utilizing information about biological processes for industrial or medical applications (5~. The five examples that follow are illustrative but not inclusive of all areas where electrochemical phenomena represent an essential component. Mechanism of Enzyme Catalysis It is possible to carry out investigations of the electrochemical properties of proteins and enzymes in biological oxidation-reduction reactions in the native state. Highly significant is the fact that there is sometimes direct exchange of electrons between the protein molecule's active center and the electrode. The thermodynamics of the redox centers have been evaluated electrochemically with the use of indirect coulometric titration. The mechanisms of such electron transfer reactions, however, are not always obvious. Primarily, the role of the protein surface, and hence the pathway of electron transfer from the electrode to the redox center, is not well understood. Understanding of such phenomena will be quite valuable in resolving more difficult questions on the mechanism of electron transfer between redox centers when those centers are not directly accessible to an electrode. Model studies are needed for dioxygen and dinitrogen metabolism, cytochrome P-450, neuroactive substances, and redox chemistry of sulfur and selenium.' The use of complementary methods such as surface-enhanced Raman spectroscopy to probe interracial interactions or proteins on electrodes would represent an important contribution. The technological incentive for this work arises from the possibility of such electrodes serving as energy converters or for highly specific electro-organic synthesis. Neuroscience Proteins are major components in dendritic nerve membranes and may exhibit electroactivity i.e., the characteristic of being switched between two states of differing ionic conductivities. Such electro- activity is interesting because the electricity of the nerve impulse, the unitary basis of information encoding in neural systems, is generated in the dendritic membrane, which is composed of electro- chemically active proteins in a lipid bilayer. Thus' by interacting with neuroscientists in the investigation of neural information coders), electrochemists may make fundamental contributions to the molecular elucidation of the human brain and the nervous systems of other major animal species (6~.
49 Technological applications emerging from such efforts include energy-transduction and -amplifying devices, information encoding devices for artificial intelligence systems, in vitro devices for sensing oxygen and pharmacological agents with membrane-immobilized proteins, and interface devices for organ or whole-body chemotherapy by metered drug release. These key scientific advances are needed in this area: · Understanding of how complex ligands (e.g., messengers, drugs) affect selectivity and sensitivity of ionic permeabilities of protein membranes, films, or lamina Improved film prototypes, such as conducting polymers involving polypeptides, which might represent improved hosts for electroactive protein insertion, as well as the characterization of a larger number and variety of such proteins in order to improve knowledge of structure- activity relationships, including the contribution of the protein to the permeability-regulating capabilities of the laden film or membrane · Better understanding of deterioration of ionic permeability, usually associated with unwanted protein adsorption, in order to to design synthetic systems that retain for practical periods their desired capabilities Cell Fusion Cell-to-cell fusion can be achieved with the aid of electrical stimulation (7-10~. Several techniques have been demonstrated in which an electric field is applied for a short duration to point- adhering (or agglutinated) cells, upon which fusion is immediately induced. The fusion may be achieved by a single DC pulse, by a series of pulses, or by gentle AC dielectrophoresis of a cell suspension. Electrofusion has been successful in all types of cells tested to date, including microbe and plant protoplasts, mammalian cells, and sea urchin ova. One can (a) fuse unlike cells to create hybrid cells; (b) fuse like cells to form larger entities such as giant cells 100 to 1000 times the volume of individual unit cells; and (c) help drive external objects or chemical agents such as DNA into cells. The mechanism of fusion is not understood. It is known that when application of an external electric field causes the potential difference across the separating membrane to reach a certain threshold value, the membrane becomes reversibly transformed from the rest state to a fusion-susceptible state, particularly in the contact zone between adjacent cells. The membrane excitation in a broad sense is observed
50 usually in milliseconds in animal cells and in seconds in plant cells. In the fusion-susceptible state, the cell membrane or lipid bilayer becomes more permeable to ions and macromolecules. ~ In addition, the emergence of a protein-free domain occurs by lateral movement of proteins away from the contact zone of the membranes between two cells. The reversible electroporation of membranes at the contact zone leads eventually to fusion. The electrofusion technique is a significant new tool for research and production of controlled systems in the life sciences. The study of electric-field-induced membrane and cell phenomena on a molecular level will contribute to fundamental understanding both of cell-to-cell fusion and of membrane structure and function. In Vivo Monitoring In vivo measurements of chemical substances can be used to provide a great deal of information concerning the regulation, metabolism, and actions of various substances inside living organisms. Chemical sensors based on electrochemical techniques are well suited for this applica- tion, because they can be miniaturized so that minimal damage is caused to the tissue to be probed. These electrochemical sensors can be used to measure the distribution and concentration fluctuations of endogenous substances or to study events in vivo such as drug partitioning between different phases. Ton-selective electrodes with tip diameters in the range of 0.5 to lO,um have been developed for ions such as potassium, calcium, and chloride, and these have been used to study the distribution of these ions in both the extra- and intra-cellular fluid. These electrodes are used in the potentiometric mode, and the specificity is established by using a selective membrane that is only permeable to the ion of interest. Voltammetric techniques have also been useful for in viva measurements; the most widely used is the oxygen electrode, which incorporates a polymer film that is only permeable to oxygen. Electrode surfaces that have been properly modified with bioreactive layers (enzyme, antibody, receptor) can provide access to the in vivo investigation of biologically significant materials. Such devices offer simplicity, low cost, miniaturization, automation and high sensitivity. Key research areas include Discovery of nolv,mer coatings that- maintain sensitivity. promote O _ _ _ _ _ ~ _ _ _ _ _ _ _ _ selectivity, protect the electrode from the biological fluid, and provide a biocompatible surface to the measured system · Development of new and improved immobilized enzymes to increase the scone of substances that can be detected by such techniques
51 · Investigation of in viva environments with the use of very fast electrochemical techniques for the elucidation of biologically significant kinetic processes Electrophoresis Electrophoresis is defined as the transport of electrically charged particles in liquid media under the influence of a DC electrical field. In these techniques, ionic constituents separate either as a function of their different rates of migration or by approaching zero mobility at different locations in an equilibrium gradient (11~. One of the most important applications of this spectrum of techniques is the separation and analysis of complex mixtures of biological origin in particular peptides, proteins, and nucleic acids. At present, two- dimensional gel electrophoresis, combined with sophisticated computer image analysis, is capable of resolving several thousand proteins among the products of a given cell type (12~. The most important applications of electrophoresis are in molecular biology and medicine where, for example, the study of inherent variabilities of serum proteins has produced a new branch of genetics, and the discovery of hemoglobin variants in several anemias has introduced the notion of molecular diseases. Electrophoresis has also greatly facilitated sequencing of nucleic acids, the clinical diagnosis of protein dyscrasias, the measurement of isoenzyme distribution, and the classification of lipoproteinemias, among others. In analytical applications the fluid is entrapped in a matrix, and visualization of the electrophoresed one- and two-dimensional patterns is done by staining, biological assays, or autoradiography, while data analysis is typically performed by densitometry (11-13~. Large-scale electrophoretic chambers (14-16) are currently being investigated for fractionation and purification of pharmaceuticals and other fermentation products on an industrial scale. COATINGS AND FILMS The need to modify the electrochemical properties of electrode- solution interfaces has led to the development of a wide range of coatings. The industry that coating technology supports has multibillion-dollar annual sales and includes areas such as paints, enamels, electrodeposits, and conductive polymers. As a result of advances in the fields of surface modification, surface character- ization, and adhesion, a revolution is occurring in coating technology. In many cases it is now possible to design coatings having desired
52 chemical and physical properties for specific applications. The following examples of this technology illustrate the advances that have been made and identify future opportunities. Protective Coatings Protective coatings are used extensively on metal or semiconductor surfaces to isolate them or limit access of an aggressive environment (17,18~. Frequently these coatings are multilayered and complex in structure, as for example in automobile paints. In this case, the innermost coating is either hot-dipped or electrodeposited zinc ("galvanizing"), over which a zinc-rich polymer-chromate undercoat is placed. The decorative top coat provides a physical barrier to the transport of water and ionic species. It is important to note, however, that protection is achieved electrochemically by the galvanic action of zinc on steel and by the inhibiting action of chromate toward oxidation. Protective coatings are not without their problems, as any owner of an automobile in a salty environment will recognize. A major problem is disbonding of the coating from the underlying metal; this phenomenon has been attributed to penetration of water followed by corrosion (19~. However, the ease with which the coating separates from the underlying structure depends on the type and strength of adhesion (19~. Current technology is based largely on physical adhesion, in which the interaction is largely van der Waals in nature. However, the development of surface modification techniques is introducing opportunities for covalent interaction and hence greatly improved adhesion (19~. For example, silanization may be used to form covalent interactions between surface hydroxyl groups and organic coating materials, as follows: R' R s' OH OH O ~ O ~ ~ + RR' Si(OCH3)2 | | + 2CH3OH /~\ /~/ Substrate Substrate If R and R' are polymerizable groups, it is possible to form covalently bound polymeric coatings that have far superior adhesion than their physically adsorbed counterparts. Adhesion is also a major problem for coatings in the microelectronics industry. For example, polyparylene films, which are used extensively
53 to protect integrated circuits, are formed by vapor deposition- polymerization techniques: CH2 4) CH2 Sublimation CH2 ~ CH2 Surface Polymerization . ~ · CH2 =0~ CH2 ~ 2 {> 2 3; Adhesion is due to van der Waals interaction with the surface, and, while polyparylene films exhibit excellent chemical stability, poor adhesion continues to be a major technological problem and limitation. Surface modification techniques, which result in covalent interaction between the polymer film and the surface, might greatly improve adhesion and hence render polyparylene films more protective. Conducting polymer films are also being extensively researched as protective coatings (20~. Using films of the right electrical and electrochemical characteristics makes it possible to polarize the underlying metal to achieve either cathodic or anodic protection. This exciting possibility of polymer "galvanizing" may well blossom into a major electrochemical industry if sufficient support is available to overcome the scientific and technical barriers that exist. One barrier that must be overcome is the oxidative degradation of the coating itself-a phenomenon that parallels the corrosive degradation of zinc in the case of classical galvanized steel. Electrophoretic deposition is used extensively in the automobile industry to form undercoats on car bodies and in many other applications. The successful application of electrophoretic deposition relies on choosing the correct environment, particle size, and system chemistry to achieve coatings of the desired properties. Of critical importance is the ability to determine and control surface charge and the structure of the electrical double layer. Probably the most extensive coatings employed in electrochemical systems are passive films that are formed on metal and semiconductor surfaces (21,22~. These anodic films are responsible for the corrosion resistance of reactive metals, such as Fe, Cr. Ni, Ti, Zr, Zn, Cu. Sn, and Al, among others, in aqueous environments as well as for the operation of various electrochemical devices (e.g., electrolytic capacitors). Decorative coatings on aluminum, titanium, and zirconium are also formed anodically, with those for aluminum being very highly developed. The principal limitation in the knowledge of the growth and
54 breakdown of passive films is the absence of a good theoretical basis for understanding the mechanisms of various processes that occur at the interface. This status reflects the lack of in situ techniques for studying the growth and breakdown of passive films. The breakdown of passive films and their inability to protect underlying metal structures is responsible for a significant portion of the corrosion losses incurred by society (discussed in Chapter 3~. Better understanding of passivity and of those factors that lead to passivity breakdown and localized corrosion would exert high economic payoff. For example, new corrosion-resistant alloys having passive films that are resistant to breakdown (e.g., the nickel-based "superalloys") may open up whole new technologies (high-performance gas turbines, advanced steam generators) on which multibillion-dolIar industries develop. Electrocatalysis Possibly the single greatest effort in electrocatalysis has been invested in reducing the overpotential for oxygen reduction in aqueous solutions at fuel cell electrodes (23~. Thus, transition metal macrocycling complexes (e.g., porphyrins), platinum on carbon substrates, and metal oxide coatings have all been investigated and have led to significant advances in fuel cell technology (24~. However, oxygen reduction electrocatalysis is still the principal limitation in alkaline and acid fuel cell performance, so that continued investment is required to bring electrochemical energy conversion into the general marketplace. Exciting possibilities exist in the use of conducting polymer coatings, because of the possibility of stereo- and product-selectivity and because the electronic properties of the coating can be controlled (at least in principle) over a much wider range than in the case of inorganic semiconductors or metals. With respect to stereo-selectivity, the concept is to include surface groups on the polymer coating that will interact with the reactant to produce a surface complex of the desired configuration. Because of the inherent asymetry of an interface (electrons and electrophiles approach from different sides), it is possible to carry out chiral syntheses of carbon atoms that have four different groups attached to it- an achievement that would open up tremendous synthetic opportunities in electro-organic chemistry. Conductive coatings may also find extensive use for product- selective synthetic electro-organic applications. Thus, conductive coatings might be used to affect oxidation or reduction of specific electroactive centers in a molecule and not of others. Such selectivity not only would improve the current efficiency for specific products but
55 also might change cell design criteria so that higher yields and hence lower production costs could be achieved. These possibilities have hardly been explored and clearly could have a significant commercial impact. Microencapsulation Microencapsulation techniques are now being actively developed along directions that will have an important impact on electrochemically based industries. Possibilities include the masking of particulate surfaces to prevent specific (and undesirable) reactions as well as the controlled release of electroactive components or inhibitors into the environment (24~. The following examples illustrate these techniques. It is now recognized that "chalking" of paints (e.g., on automobiles) is due principally to photoelectrochemical processes that occur at the surface of the e-type TiO2 pigment particles (25~. Thus, the absorption of photons generates electron-and-hole pairs; the electrons reduce oxygen from air and the holes either oxidize the polymer binder directly or lead to the formation of reactive chemical species (OH radicals, H2O2, and organic peroxides) that subsequently react with the organic matrix (25,26~. A complete understanding of this photo- electrochemical degradation, which is responsible for millions of dollars in losses annually, requires a detailed knowledge of the electrochemical properties of semiconductor systems. ELECTROCHEMICAL CORROSION The Panel on Electrochemical Corrosion was formed to conduct a critical evaluation of issues and opportunities. This section summarizes the conclusions and recommendations reached by the panel. A more detailed report will be issued separately (A Plan for Advancing Electro- chemical Corrosion Science and Technology, NMAB Report 438-2, 1987~. The panel addressed three general topics: corrosion research and engineering, research on advanced materials, and dissemination of information. The first of these topics focuses on fundamental understanding of corrosion processes, on utilization of measurements and understanding in the engineering systems analysis of corroding systems, and on life prediction. The second examines corrosion of emerging classes of materials. The third addresses education and the transmittal of information on corrosion and corrosion control to the technical community. The panel's study led to the conclusion that a new approach to corrosion science and corrosion engineering is not only necessary but
56 possible. The required capabilities are becoming available in the scientific ability to model surfaces and interfaces, in the electro- chemical and surface science techniques for studying interfaces in situ, in the computational facilities for modeling, and in materials processing technology. The panel concluded that an approach is required that builds on existing multidisciplinary capabilities of individuals and institu- tions. Further, this approach must provide a mechanism that integrates multidisciplinary activities into a framework that brings coherence to complex phenomena and yields a comprehensive basis for understanding them. Six central recommendations were identified on theory and modeling, experimental probes, lifetime prediction, investigation of advanced materials, multidisciplinary efforts, and education. · Theory and Modeling: Greater emphasis on modeling and theory is recommended for both elementary corrosion processes and their interactions in complex macroscopic systems. Given the opportunities and need in the next decade for this field to adopt advances made in other disciplines, the panel concluded that greater support of theory and modeling is justified even if the total support of this field remains constant. Two complementary areas for theory and modeling have been identified-elementary processes and macroscopic systems. Regarding elementary processes, new theoretical approaches for characterizing electrolytes are in hand and are being applied to dielectric-solvent surfaces. Just emerging are theoretical treatments for the physics of electrons at metal-electrolyte interfaces. The incorporation of understanding from both these areas in theories to describe the elementary processes at metal-electrolyte interfaces is possible, even for the complex interfaces encountered in corrosion systems. Extension of this work to include interracial films will provide a fundamental physical understanding of metallic corrosion capable of predicting corrosion behavior from first principles. Descriptions of individual corrosion processes can be assembled and used to predict materials degradation in macroscopic systems. However, the computations required are usually so lengthy and complex as to require access to large scale computational facilities. Expansion of this approach to the analysis and prediction of corrosion behavior on a wider scale requires the development of more efficient mathematical techniques and algorithms and of methods for simplifying the calculations without loss of significant factors. · In Situ and High Resolution Experimental Probes: The active support now given to the development of probes to measure corrosion processes in situ and with the spatial resolution needed for studying local corrosion phenomena should be continued. Of particular importance is the use of probes where possible as sensors for on-line monitoring of corrosion of components in technologically important systems.
57 Over the past decade, a revolution has occurred in the field of electrochemistry with the development of in situ and ex situ surface analysis techniques capable of resolving important phenomena on both microscopic and short time scales. These techniques should be adapted and utilized to characterize local physicochemica' corrosion events in situ. In addition, in situ techniques should be extended to provide on-line monitoring of real-world systems where reliability often requires detecting the onset and progress of corrosion phenomena (e.g., pit depth and crack length) as a function of time. · Lifetime Prediction in System Applications: Quantitative methodologies for predicting lifetimes should be developed, coupling advanced models with identification and measurement of critical parameters and with computer-based expert systems. This effort will necessitate generating physicochemical data bases to support systems analysis as well as using advances in theory and experimental techniques discussed above. A major objective of corrosion science and engineering is to permit selection of materials giving corrosion resistance compatible with system design in specific service environments. Even for the simplest case, general corrosion of metals, present lifetime prediction strategies are qualitative or nonexistent because of the lack of (a) realistic models, (b) understanding of critical parameters, (c) test data, or (~) suitable coupling between the models and the experimental results. These factors must be addressed if materials are to be selected for reliable and economic service. Currently available thermodynamic and kinetic data bases are incomplete to support quantitative modeling of many corrosion systems, particularly those where predictions of behavior under extreme conditions or over extended periods of time are desired. Because the unavailability of data limits the use of models, a critical need exists to upgrade and expand the sources of information on the thermodynamic properties of chemical species, exchange current densities, activity coefficients, rate constants, diffusion coefficients, and transport numbers, particularly where concentrated electrolytes under extreme conditions are involved. Many of these data are obtained in disciplines that traditionally have been on the periphery of corrosion science, so it will be necessary to encourage interdisciplinary collaboration to meet the need. A number of proprietary expert systems are being developed for corrosion engineering, specifically for materials selection in marine environments, in pressurized water reactor steam generators, and for high strength aluminum alloys. The availability to designers of computer-based expert systems for corrosion engineering will improve the performance and reliability of new structures and systems. Knowledge of
58 corrosion and related phenomena for specific materials under consideration for use is an important input to the materials selection process in the early stages of design, where problems can be dealt with most effectively and without compromising design intent. This knowledge is at present gained principally through practical experience and so is held by "experts~. Codifying their knowledge for wider accessibility and utility will lead to improved corrosion resistant designs. · Corrosion Resistance of Advanced Materials: The corrosion behavior and limits of chemical stability of newly developed materials should be determined as an integral part of materials development in order to indicate where more detailed modeling and experimental efforts are warranted. New engineering materials, evolved through chemical synthesis or advances in processing, require study to determine the limits of their corrosion resistance in service environments. Baseline investigations on advanced materials are a prerequisite if their corrosion properties are to be characterized sufficiently to allow them to be introduced reliably into engineering systems. For example, some metallic glasses appear to be remarkably inert and have commercial appeal. In contrast, metal-matrix composites are being pursued for structural applications but in many cases appear to lack corrosion resistance. The use of ceramics in electrochemical systems as separators, electrodes, electrolytes, and containment vessels emphasizes the importance of understanding and enhancing reliability while maintaining attractive chemical, electrical, and other properties in new service environments. · Multidisciplinary Activities and Education in Corrosion Science and Engineering: Industry, government, and academia should foster multidisciplinary research approaches. These will draw upon advances made in related fields of physics, mathematics, and electrochemistry, among others, and must build on the strengths of individual participants and facilities in these several fields. Advances in the stabilization of interfaces will benefit from enhanced multidisciplinary approaches in education, in research, and in application. Because corrosion science incorporates elements of physics, chemistry, electrochemistry, materials science, mathematics, and engineering, it is essential that scientists and engineers skilled in these disciplines be encouraged to contribute to this field, i.e., to its concepts and theories, predictive methods, and experimental techniques. The panel concluded that industry and government should provide this encouragement by expanding support of collaborative efforts. The panel further concluded that an essential part of the development of this field will be improved undergraduate and graduate education in this field in universities; this is needed to provide
s9 trained engineers and scientists capable of contributing to advances called for in efforts recommended in this report. · Instruction in Corrosion Practice: Improved education must be provided on a continuing basis to engineers respc:~sible for materials selection. A broader knowledge of corrosion on the part of the users of materials in design will result in major reductions in the corrosion- related costs of maintenance, repair, and replacement. The correct selection and usage of materials to withstand the corrosive environ- mental influences that cause degradation and failure must be based on an appreciation of these influences and the ways in which they can affect materials and structures. Such knowledge can be supplied by utilizing existing resources for continuing education and should be a part of the background of all those who are concerned with design. However, the education of engineers at the bachelor level is deemed inadequate-it will probably be limited to a single course in a materials curriculum. Efforts should be made to include more laboratory experience in corrosion in conjunction with lecture courses at this level. ELECTROCHEMICAL SURFACE PROCESSING The use of electrochemical processes for modification of surface properties is growing in response to needs for new materials and more demanding process requirements (27-32~. A wide variety of materials may be modified in this manner, including metals, semiconductors, and dielectrics. Techniques for doing so include passing electrical current through solutions (electrodeposition, electroforming, electrogal- vanizing, electropolishing, anodizing, electromachining) and ionized gases (plasma etching, plasma-enhanced chemical vapor deposition) and exposing to aggressive solutions (etching, chemical milling, electroless plating). Industries based on these phenomena represent one of the largest groups of electrolytic technologies on the basis of value added, as indicated in Chapter 3. Many aspects of the fundamental processes that occur during these operations are closely related to other important technologies such as mineral processing, electrorefining, battery charge and discharge phenomena, and corrosion. Current applications fall into four general categories: · Corrosion protection, including deposition of metal coatings such as electrogalvanizing of steel with zinc and electrophoretic deposition of insulating films, both of which are industry standards for protection of automotive bodies
60 · Imparting technological properties such as conducting layers and contacts in microelectronic circuits, wear-resistant bearing surfaces, low-resistance, low-pressure electrical contacts, high- frequency waveguide surfaces, and special morphological, textural optical, chemical, electronic, dielectric, tribological, and catalytic properties · Conserving expensive or strategic materials by coating inexpensive substrates with thin layers of gold, palladium, cobalt, chromium, etc. · Imparting decorative properties that increase product value Emerging new applications are expected in the near term for novel materials, microelectronic devices, sensors, and new methods for creating machined parts. Such applications will demand sophisticated new technology, including precise control and purity. The attainment of these capabilities would bring significant economic benefits. The following are some of these new applications: New materials formed by alloy plating to produce thick amorphous materials having special properties of strength, hardness, magnetism, etc.; to contribute to corrosion resistance (high-rate deposition of stainless steel and of alloys that are solderable and paintable); to replace gold for electronic contacts; and to produce conducting polymer coatings formed in situ by electropolymerization Novel coatings formed by electrophoretic deposition of ceramics, glasses, conducting polymers, and high-temperature polymers Composite materials fabricated by codeposition of particulates (MoS2 in metal for lubrication, diamond or SiC in metal for wear resistance, etc.) and by layering (through periodic alternating of plating conditions) High-rate electroplating and electroforming of precisely patterned thin films as well as of entire parts now made by energy-intensive casting and machining operations Microelectronic devices formed by electrodeposition of active patterned semiconductor components at low temperature Ultrathin layers formed by underpotential electrodeposition for novel catalysts and molecular-scale materials Three-dimensional microfabrication of shaped parts by selective electrodeposition through variable thickness or variable conductivity masks or with laser-enhanced or photo-assisted pattern plating
61 While many of these achievements have been demonstrated in the laboratory, there are technological barriers to achieving these capabilities in commercial practice. These include several general processing problems such as waste disposal (development of new self-contained or reduced-toxicity systems), on-!ine automation (stable sensors for monitoring process conditions and product quality and algorithms for automatic process control), precise control of process chemistry (including purity), and control of the transport process by proper equipment design procedures (particularly for efficient high-speed plating, etching, and plasma processing operations). In addition, better understanding is needed of alloy deposition phenomena, of bath and plasma chemistry, of the role of additives and impurities, and of the structure-property relationships of coating and base material systems of technological importance. Scientific and engineering advances are needed along several specific lines of fundamental research in order to speed technological development: Ultrapure electrochemistry-Surface treatment under clean conditions often gives structures and properties that are seemingly anomalous. Studies under ultrapure conditions would provide significant new understanding of surface phenomena, including the role of additives and impurities. Solid-liqllid interface structure Investigation under rigorously controlled conditions of solid-liquid interfaces, with both in situ and ex situ characterization of components, is nearing feasibility. Improved understanding of the extended structure of both fluid and solid phases in the vicinity of the interface would be a major advance in the scientific level of this field. Surface evolution There is limited fundamental knowledge of how surfaces are created and destroyed at the atomic level. New surface science and mathematical tools should be used to develop a theory for electrocrystallization. Better understanding is needed of instability phenomena that lead to shape evolution, such as dendrites, surface roughness, anisotropic chemical and plasma etching, and patterning. Simulation of electrolytic cells Effective surface processing technology hinges on the ability to design and scale up processes in a predictive manner. Recent accessibility to enhanced computing power, combined with new modeling techniques, makes major advances feasible. Tailored properties Theories are needed for predicting the structure, composition, properties, adhesion, and uniformity of electrochemical and plasma-generated surface films in order to control microscale phenomena.
62 Discovery of new materials-Innovative techniques of surface modification should be encouraged to identify new systems of technological interest. The opportunities for such discovery are truly excellent. The field of electrodepositiox~, plasma processing, and allied electrochemical surface-modification techniques has significant economic and strategic value. Nevertheless, this field is treated as if it were noncritical in nearly every industrial, academic, and federal program. There is no single technology base to serve as a focal point for identifying key technological barriers, there is no well-defined research sponsor, and there are few educational facilities for development of new researchers or programs for retaining current practitioners. Equipment manufacturers tend not to be equipment users, and both groups tend to be highly secretive so that they impede each other as well as the advancement of the field. As a consequence of these inadequate institutional arrangements, the research needed for advanced electroprocessing of materials and their surfaces is not adequately pursued in the United States. The federal government can and should play a major role in this field. The committee therefore recommends that a detailed assessment be made of scientific and technological opportunities and routes to their realization in the area of electrochemical surface processing. The following research areas hold high promise for advancing technological growth in the near term: · Electroplating under ultrapure conditions · Transport and reaction phenomena during high-speed processing · Invention of new processing conditions and discovery of new unique materials fabricated by electrochemical surface processing · Theory for electrocrystallization at solid-liquid interfaces MANUFACTURING AND WASTE UTILIZATION Metal and industrial mineral production is a major manufacturing activity in the United States. The primary metals industry is an important component of the economy. Larger than the production of industrial chemicals, the primary metals industry approaches the manufacturing of motor vehicles in gross product dollar value, and it exceeds both chemicals and automobiles in total employment. Projections to 1990, given in Figures 5-4 and 5-5, show only a slight drop in
63 Motor Vehicles Primary Metals Electronic Components Industrlal Chemicals Mlning and Quarrying Crude Petroleum and Gas ~\~\\~\~\~ : ~\~\~,~ _ ////////////// ////// .. .. ....... ....... .. , ,..: .. :.: :. 1 ,....,.. ...,..,....a ~\\\\\1 ///// ? -'Z ::' . ... :...! :.~; .~ : :s A\\\ /// ~ at' : 1 :::::.,: .:.:.,,,:.,.,.:] \\\ Averaged over years 1990-1 995 . ~3 (projected) [ ~ 1980-1985 ~j3 1970-1975 I I . I l ~l I ~I 0 20 40 60 80 100 120 BILLIONS OF ALLIS FIGURE 5-4 Gross average value of U.S. industrial production for the years shown (33~. Primary Metals Motor Vehicles Electronic Components Mining and Quarrying Industrial Chemicals Crude Petroleum and Gas % of Total /////////A \ ~/,//~/~/9 \\\\ ./~///A Gil\\\\\\\\\\] 23 25 33 23 24 29 21 18 12 //~//~/~/1 14 \\\\\\\\\\ Averaged 12 ~/~71 years 10 ^\~j~\3 1990-1995 =3 ~ o (projected) ////~//A 1980-1985 ~9 _ 1970-1975 ~4 I I ~I I I I I I ~1 1 1 0 200 400 600 800 1000 1200 . . THOUSANDS OF JOBS FIGURE 5-5 U.S. industrial employment (thousands of jobs averaged over the years shown).
64 primary metals relative to both motor vehicles and electronic components and indicate a high level of importance to the economy for the foreseeable future (33~. Technological advances in minerals processing could result in lower capital and operating costs; these may offset other increases as the U.S. industry of necessity moves to ores with lower metal content and greater complexity. Such advances are important in ensuring the future competitiveness of the U.S. industry. Electrochemical reactions occupy an important position in mineral processing and are likely to maintain this key role for a long time. Other process alternatives exist for the smelting of the less reactive metals (zinc, lead, etc.), but virtually all the nonferrous metals (alkali and alkaline-earth metals, magnesium, etc.) are directly or indirectly produced and/or refined by electrolytic processes or by a product of such processes. The capability of electrolytic processes to extract metal values from very dilute solutions coupled with high selectivity offers some unique advantages in extractive metallurgy and waste treatment. Another unique feature of electrolytic reduction and deposition of metals is the promise that it may be possible to make near-net-shape parts, which could result in the omission of numerous costly intermediate thermal and mechanical processing steps. Significant progress can also be made in the application of electrochemistry to the development of sensors that could permit the use of advanced microprocessor control systems for optimization in real time without interruption. An example of this is the potentiometric probe now routinely used for hot metal analysis in steelmaking. Such techniques may be extended to gas and molten metal analysis as well as to coupled electrochemical reactions (recently recognized to be important in hydrometallurgical operations) that can best be studied by precise electrochemical methods. Current Applications The principal industrial electrolytic processes in the order of percentage consumption of total U.S. power production are aluminum, chlorine, magnesium, sodium, sodium chlorate, zinc (electrolytic), sodium perchlorate, copper (electrolytic), and manganese. This group, along with electro-organic synthesis and other processes, consumes about 6 to 7 percent of the total electrical energy generated in the United States (34~. Of the group mentioned, aluminum and chlorine consume 66 percent and 28 percent respectively, i.e., between 5.5 and 6.5 percent of the total energy produced (35~. Improvements in the power efficiency of these two, then, would have the most significant impact on energy conservation. Much research effort has been expended by these industries, resulting in a significant reduction in their electrical energy requirements; but at 42 to 48 percent for aluminum and 58 percent for chlor-alkali, power efficiencies are still poor. Power
65 efficiencies are similar for all of the electrolytic processes. There is room for technical advances, both incremental and revolutionary, in all of these processes. Incremental advances are cost-reducing improvements in existing technology and tend to have a relatively small impact on cost or international competitiveness. Revolutionary advances lead to "leapfrog" technology, which can have a large impact on capital and production costs but may take years to develop and long periods to become widely adopted. Electrolytic Processes Based on technology concepts known today but not yet reduced to practice, energy savings of 30 to 40 percent seem to be possible for both aluminum and chlor-alkali and in all probability for the other industrial electrolytic processes mentioned (36~. Bringing this savings to commercialization would therefore be worthwhile from an energy conservation point of view. However, it seems apparent (at least in the case of aluminum extractive metallurgy) that technological development on existing processes that might offset the inherent advantage of the foreign producers is unlikely to be reduced to practice because of the high financial risk, high capital requirements, and the likelihood that foreign producers would restore their advantage by quickly adopting the new technology. It also seems unlikely that a totally new, more efficient process is imminent, since large sums of money have been spent over the past 25 years by aluminum producers in search of a process that would override the inherent difficulties of electrolysis (low space-time yield and low power efficiency) with only moderate success. Candidates such as bipolar chloride electrolysis and direct reduction with carbon have been developed through pilot scale, but the high cost of capital and the low probability of retaining advantages over foreign producers long enough for an acceptable return on investment preclude their commercialization in the foreseeable future. It seems prudent, therefore, to reduce the research and development effort on the existing processes to incremental, low-capital-requirement improvements (in order to delay the demise or off-shore siting of these processes) and to direct a major portion of available research resources to the leapfrog-type developments described under "Emerging Technologies" later in this section. One possibility is the development of innovative technologies that could create new, competitive processes for the production of U.S. metal and mineral requirements based on ore or waste stream resources available only in the United States. In the case of aluminum, processes should be sought in which aluminum is a by-product, or at least one of a group of metals and minerals produced from a single ore, so that capital charges could be distributed over a variety of products instead of a single one. It is unreasonable to
66 extract one valuable constituent from an ore and then have the costly problem of discarding the waste in~an environmentally acceptable manner. Automotive Applications Electrochemical phenomena are deeply entwined in the development of the automobile. Lead-acid batteries have long been the only viable choice for starting of internal combustion engines. Although the lead- acid battery is one of the heaviest possible choices for both metal and electrolyte among commercially available batteries, its reliability has made it the portable power source of choice. Remarkable improvements in performance have been made during the past decade. Although nickel-zinc batteries give better performance with regard to higher energy density and cold starting power than lead-acid, replacement has been slow because of their higher cost. A major problem in automotive vehicles is corrosion control, particularly in the northern areas of the country where salts are used to melt road ice. Protection of exposed steel from corrosion is accomplished by electroplating with copper-nickel-chromium films, particularly on trim and finish moldings. Electrophoretic painting of body panels is used industry-wide, and electrogalvanizing to protect steel under paint is soon to become a standard practice for all vehicles. Electrochemical accelerated tests for the integrity of coatings and for the determination of the concentration of coolants are used extensively. Applications to the power plant itself appear in the form of electrolytically produced lead-tin coatings on specialty bearings and hard chromium on wear surfaces such as pistons. Electrochemical machining is used for close-tolerance requirements on intricate parts that are hard to machine by conventional means, although such processes are limited to specialty applications. In this case significant benefits can be derived for the domestic industry through electro- chemical research aimed at lighter, corrosion-resistant, lower cost vehicles in the short time frame as well as fundamental understanding that identifies major breakthroughs for longer range competition. Electro-Organic Synthesis There are about 30 electro-organic synthesis processes thought to be in production and 100 additional ones that have been demonstrated to be feasible on bench scale. In recent years electrocatalysis has been shown to have significant promise in such reactions; in many cases, these simulate biological processes and show equivalent selectivity.
67 The extensive interest in these electrochemical processes is shown in Table 5-3, which gives a sampling of processes in production along with the diversity of developers, which range widely across nations, academia, and industry. Table 5-4 is a sampling of the processes shown . to be feasible but not yet commercialized. Recently, more attention to electrocatalysis and to better under- standing of the fundamentals of electrochemical reactor design have permitted more accurate economic analysis and the proper selection of products for this technique (37~. Some ~ to 10 years ago mathematical modeling of electrochemical reactors transcended empiricism and permitted the generation of economic models to guide process selection Nearly development. With a sufficiently complete data base, software packages are becoming available that generate a complete reactor design with the economics of raw material, labor, and utilities incorporated to predict profitability if a probable selling price is known. Although simple planar cell designs such as those used in chior- alkali, aluminum, and magnesium production are still the industry workhorses, new cell designs are being developed based on the use of porous and fluid-bed electrodes. The invention and engineering development of three-dimensional porous electrodes having high reaction rates per unit volume have permitted vastly improved cell designs and have reduced capital costs enough to make some electrolytic processes competitive, even with rising energy costs. E· ~ ~ ~ ~ merging 1 ecnnologles It is generally recognized that the technology employed in electrolytic mineral processing and extractive metallurgy is somewhat primitive. Because of the high internal resistance of electrolytic systems and the problem of back-reaction when the anode-cathode spacing is too close, current densities must be kept low. This results in low space-time yields (tons of product per cubic meter of reactor) and poor power efficiency. For instance, the average space-time yield for pyrolytic processes, including the iron blast furnace, is about 3 x 10-3 tons/m3/minute, whereas the space-time yields for electrolytic refining of copper is only 5 x 10-4 tons/m3/minute and for aluminum production S x 10-5 tons/m3/minute (one and two orders of magnitude poorer, respectively). By the criterion of productivity per unit of floor area, the pyrolytic iron blast furnace is very efficient at 20 tons of metal/m2 of floor area per day, whereas the electrolytic processes are quite poor, electrowinning of copper showing only 1/10 of a ton of metal/m2 of floor area per day. Many extractive metallurgy operations fall into the latter category (Hall cells, flotation
68 TABLE 5-3 Some Commercial Electro-Organic Processes Product Reactant Developer Adiponitrile Acrylonitrile Monsanto; Asahi Chemical; Rhone Poulenc Aminoguanidine Nitroguanidine ICERI* Aniline sulfate Nitrobenzene ICERI Anthraquinone Anthracene Holliday Benzidines Nitrobenzene ICERI Bromoform Ethanol ICERI Calcium gluconote! Glucose/iactose Sandoz India; Chefaro; R. O. lactobionate Herdom Poland 2 5-Dimethoxy- Furan dihydrofuran BASF Dimethyl sulfoxide Dimethyl sulfide Gianzatoff; AKZO; Petroles ~ Aquitaine Dodecenedicarboxylic Unknown acid Fluorinated carboxylic acids Fluorinated methanesulfonic acids Alkanoic acid fluorides Bis-fluoro-sulfonyl 3M methane Glyoxylic acid Oxalic acid Hexahydrocarbazole Tetrahydrocarbazole BASF Hexafluoropropylene Hexafluoropropylene Hoechst oxide Japan 3M; Dai Nippon Rhone Poulenc; Streetly Chemical; Japan
69 TABLE 5-3 (continued) Product Reactant Developer Maltol/ethyl Mattel -Methoxybenz- aldehyde -Methoxybenzy] alcohol Furfury! alcohol -Methoxytoluene -Methoxytoluene BASF Otsuka; BASF; ICERI 2-Methylindolene 2-Methylindole Holliday Naphthaquix~one Naphthalene Holliday; ECRC**, B.C.; Research Inst., U. of British Columbia p-Nitrobenzoic p-Nitrotoluene ICERI acid Salicylaldehyde Salicylic acid USSR; ICERI Sorbitol/mannitol Glucose Atlas Powder Succinic acid Maleic acid ICERI Tetraalkyllead Alkyl/Briqnard, Pb Nalco Chemical Tetradecanoic Monomethyl suberate Soda Aromatic · acl~ . o-Toluidine acid o-Nitrotoluene ICERI ~- *ICERI Indian Central Electrochemical Research Institute **ECRC- Electricity Council Research Center
70 TABLE 5-4 Some Electro-Organic Processes Shown To Be- Feasible on Bench Scale but Not Yet Commercialized Product - Reactant Developer Aminobenzoic acids Nitrotoluenes - Electricite de France Carbonates Ethyl oxalate Royal Dutch Shell Ethane tetracar- boxylate Malonic diester Monsanto Ethylene glyco! Formaldehyde Electrosynthesis Co. Fluorinated and Alkanes, Phillips Petroleum partially chIoroalkalies, fluorinated alkanoic acid alkanes, fluorides - chioroalkanes, and carboxylic acids Geraniol/Nerol N,N-Diethyl-o-geranyl University of New Castle (neryl)-hydroxyl amine Oxalic acid Carbon dioxide Dechema Institute; University of New Castle Sorbic acid Butadiene/acetic Monsanto acid Tetramethyldithioram Dimethyl dithio- DuPont disulfide carbonate o-Toluidine o-Nitrotoluene University of Eindhaven circuits) and are limited to a few feet of height, resulting in sprawling plants that cover many acres with relatively high costs in terms of land, foundations, buildings, and utilities. The present state of the art therefore constrains the use of electrolytic processing for extractive metallurgy to three broad categories-those products that can be made by no other known method with available containment materials, those with sufficiently high density or markup value to offset the required capital and power costs, and those processes where more than
71 one product is produced, permitting the costs to be distributed over a larger value base. The new technologies suggested in this section are aimed at one or more of these problems. Coproduction of Metals and Valuable Anodic Products Although the need to extract metals more efficiently from complete low-grade ores has generated increasing interest in sulfate electro- winnir~g systems utilizing an anode reaction other than the oxidation of water, little attention has been given to coproduction methods in which the anode reaction also produces a valuable product. In coproduction the energy used can be charged to two products rather than one. One illustration of this is a recent Bureau of Mines study (36) that showed that copper electrowinning can be combined with the electro- chemical production of sodium perchIorate using a cationic membrane, significantly decreasing the energy requirement for each product. Sodium hydroxide, hydrogen, and chlorine can be produced concur- rently in a cell where a sodium chloride-zinc chloride mixture is separated from sodium hydroxide with a beta-alumina diaphragm. In such a cell (which to date has simply been bench tested), pure molten sodium hydroxide and dry chlorine are produced. Because of the higher temper- ature, the cell operates at lower overvoltage and ohmic loss than the conventional aqueous electrolytic processes (38~. Inorganic Chemicals Chlorine, hydrogen, and ammonia may be coproduced from ammonium chloride using a beta-alumina diaphragm separating a sodium chloride-zinc chloride mixture on the anode side and an ammonium chloride-sodium chloride-zinc chloride mixture on the cathodic side, where ammonia and hydrogen are produced (38~. None of the thermochemical or hybrid cycles for hydrogen production are as economical as the electrolysis of water, which is still too costly for fuel cell application (39~. The high overvoltage of such a cell could possibly be overcome by operating a high-temperature system in which a small amount of water dissolved in molten sodium hydroxide is electrolyzed to produce hydrogen and oxygen. In preliminary bench tests current efficiencies (anodic and cathodic) were high and overvoltages low because of the high operating temperature of 330°C (38~. It is conceivable that a high-temperature system similar to the one suggested earlier may make the electrolysis of water a viable source of hydrogen for fuel cell applications.
72 An old idea of fixing nitrogen from the air by means of an electric arc is being considered for fertilizer production in remote Third World areas where electric power is available from new hydroelectric plants and food production is a major concern (40,41~. Electric Power Production Many processes can be designed to produce chemicals and electricity in electrogenerative cells, utilizing waste material in some cases. In electrogenerative cells electricity is produced rather than consumed, providing a by-product while eliminating the capital investment for the power supply. Research and economic analysis should be able to uncover some processes that could be commercialized in the future (42~. Burning hydrogen and chlorine for the chlorination of hydrocarbons may be carried out either in a HC1 aqueous cell or fused metal chloride cell, simultaneously producing electrical energy and marketable chemicals by the cell reaction. Nitric oxide may be electrogeneratively reduced in an electro- chemical cell to generate by-product electricity while producing ammonia and eliminating a polluting effluent gas stream (42,43~. Waste Utilization Electrochemical separations have not found wide commercial application. The cost per unit mass cannot, in general, compete with conventional techniques such as distillation or extraction. However, for high-specific-value components the selectivity available with electrophoresis or enhanced membrane transport often makes these the processes of choice. An analogous situation is found with contaminant removal. In this case, however, the value is mainly in the removal of a species. Thus, a technique such as electrowinning, long used to remove precious metals from solutions of low concentration, can be profitably applied to the removal of trace quantities of hazardous components. Potentially treatable effluent streams are in either gaseous or liquid form. Application to date has been almost exclusively to liquid streams, mainly because of the history of precious metal recovery for profit. Metal-bearing liquid wastes from plating, galvanizing, dipping, cleaning, and stripping operations, as well as from electronic component manufacturing, may contain dangerously high levels of chromium, nickel,
73 lead, zinc, and other metals. These wastes must be treated before being discarded. Heavy-metal recovery from such streams is accomplished either by electrowinning or electrodialysis. The former method is essentially electroplating, but from a very dilute stream. In one process of this type, a succession of separation, chemical reactions, and electrowinning is used to extract the metals according to their decomposition potential depositing copper, chromium, zinc, iron, and nicked separately in sequence. The economics have yet to be worked out, and more research will be needed before it is commercially viable (44~. In electrodialysis, an alternating stack of cationic- and anionic-selective membranes is employed in an applied field to produce a concentrated waste stream along with cleaned product water. Gas-phase purification using electrochemistry has been limited mainly to flue gas desulfurization. These techniques have not yet found wide acceptance because of their low energy efficiency or insufficient development (45~. Electrochemical treatment of liquid and gaseous waste streams is discussed separately because their economics, acceptance, and R&D needs are quite different. Systems for handling liquid effluents are well developed. Commercial units are available for recovering copper, lead, cadmium, nickel, and zinc. Most operate on the basis of electrowinning; that is, the metallic cations are plated out of solution onto specifically designed cathodes. Very high surface area is necessary to avoid costly concentration overvoltage. This has led to various types of porous electrodes, including carbon-fiber electrodes. The last, known as the HSA reactor, has been the subject of a number of test programs (46~. This reactor is now marketed commercially and seems capable of treating most normal plating waste streams (47~. A study recently completed for the U.S. Environmental Protection Agency shows this is indeed the case if the plating operation is run with a closed-loop first rinse that is demetallized in the reactor followed by a final rinse that is then dilute enough to be directly discharged. To collect the metal, the carbon-fiber electrode is made the anode in conjunction with stain- less steel cathodes. The metal is then mechanically removed from these flat plates. Other designs are also available, most using flat or rolled cathodes that do not require current reversal for metal recovery, with capital costs ranging upward from $5000. The anodic reaction is generally oxygen evolution from Ti-Pt electrodes. With cyanide solutions, however, the cyanide anion is oxidized to acceptable levels at the anode. Other anodic reactions, such as oxidation of SO2 or organics, have been tested but have not been commercialized (48~. The alternative to metal recovery is precipitation, where a sludge is formed and then disposed of in a landfill. This seems to be the choice of most plating operators. The main reason is that a recovery
74 system is typically designed for a specific metal in a specific concentration range. Shops may have five or six plating rinse waters to treat. Although the operating cost is dominated by electricity, tYDicallY 1 to 2 kWh/lb. and is actualiv less than the value of the ~ _ ~ ~ ~ ~ , ~ ~ , ~ ~ ~ ~ recovered metal, the caplta1 expenditure required t~or several recovery systems is viewed as intolerable. As long as a simpler solution is available-i.e., sludge dumping it is unlikely that widespread application of metal recovery will be seen. For liquid waste, then, the conclusion is that the technology is indeed in a sufficiently advanced state. Further utilization requires a higher negative value for the waste i.e., penalties for sludge or incentives for recovery. . The situation is radically different with regard to gas treatment. One large installation for flue gas desulfurization has been operating in Japan for some time. The electrochemical step is one of scrubbing- fluid regeneration rather than direct gas treatment. However, the "Sulfomat" system is effective, even if somewhat high in energy consumption (49~. There have been several electrochemical processes proposed that destroy contaminants, generally by oxidation at an anode (50~. In most cases where oxidation is indicated, however, incineration is simpler, although there may be gas streams where incineration is clearly not in order, as with an oxidizable product component. In these cases the selectivity offered by electrochemical means could be valuable. Gas-phase electrochemical membrane transport is one of the techniques chosen by National Aeronautics and Space Administration or manned spacecraft CO2 control (51~. This technique, suggested by fuel cell development, has given rise to similar processes for SO2 removal from flue gas and H2O removal from coal gas (52~. Metallic sodium, or sodium hydroxide and sulfur, may also be extracted from flue gas by electrolysis of molten sodium sulfide (produced in the gas desulfurization process) by application of the charging reaction of the sodium-sulfur battery. This could conceivably be converted to a power-producing system if oxygen can be reduced at the cathode without severe polarization. Again, a beta-alumina diaphragm must be used to separate the sodium sulfide from the sodium hydroxide. Electrogenerative reduction of nitric oxide for pollution abatement is being investigated in mixtures of gases, which include constituents encountered in stationary power-plant effluents. It may be possible to use suitable electrochemical cells for the removal of nitric oxide from power-plant gas streams, with the possibility of recovering reduced constituents as useful chemicals (53~.
75 Gas-phase devices treating dilute components must deal with high concentration overpotentials and subsequent low current densities. Technical solutions to this problem have been found in the case of liquid treatment, as outlined earlier; similar solutions may be found for gases. Electrocatalysts for selective oxidation of gaseous components are only now receiving some attention (54~. Thus, there appears to be fertile ground for explorative research in these areas. New Automotive Applications New lithium-based and the more conventional Ni-Zn batteries may eventually replace lead-acid batteries as new technology and advanced manufacturing techniques reduce their costs. Metal-air batteries, both rechargeable (zinc) and nonrechargeable fuel-cel1 types (aluminum), may ultimately be successful as an economical primary source for short-trip transportation. The demand for increasing electronic equipment will require increased auxiliary power, which may be fulfilled by improved lithium-based and Ni-Zn systems. Greater understanding of the parameters affecting electroplating may permit controlled structures in Zn-Ni and Zn-Fe coatings on steel and improved corrosion control. Computer-controlled electrochemical techniques possess in principle the ability to accurately reflect the performance of such coatings; on-board sensors for applying the proper corrosion current for protection are being considered. New alloy coatings with controlled structures could have tribological applications on wear surfaces while permitting the use of methanol as a fuel through greater corrosion resistance. Electrochemistry offers a new era of sensor development, allowing real-time information to be used to control and inform as to the condition of a variety of important parameters involving vehicle operation. These might include analysis of the exhaust system with regard to hydrocarbons, carbon monoxide, and nitrogen oxides followed by appropriate action for control; signals related to corrosion and wear so that manual corrective action could be taken; and the composition of coolant involving the appropriate alarm when the concentration of undesirable constituents exceeds a safe range. Information could also be available that allows the occupants' environment to be controlled to avoid stress (measured by surface electrolytes) and that indicates intoxication (alcohol) so that the vehicle's operation may be curtailed. If electrochemical power plants become economical as primary power sources, sensors linked to microcomputers optimizing the system could be used to monitor the electrode and electrolyte condition. Fuel cells may be developed for either auxiliary or primary power using methanol as the liquid fuel of choice, and in the distant future technology could evolve to synthesize hydrocarbon fuel on board.
76 High-resolution circuitry and active devices employing Langmuir-Blodgett film techniques or polymer-based transistors are being considered for the sophisticated electronics required in future vehicles. Temperature or energy balance in the vehicle could be controlled through conductive polymers or semiconductor deposits on electrochromic windows. Electro- Juminescent liquid crystals and fluorescent and electrochromic materials used for visual displays show promise for future development. Careful studies of the interaction between wear surfaces by spectroelectrochemical probes may result in surface modification to create wear resistance and design lubricants resulting in much longer · ~ flee In engines. Pollution currently resulting from the automotive industry could be controlled in principle by the electrochemical recovery and recycling of oils from both engine lubrication and machining operations as well as other chemicals involved in automotive manufacture. It may even be possible to develop a fuel cell to convert these pollutants to propulsion power. Recycling of valuable construction materials could also become commonplace to reduce both energy costs and the importation of raw materials. Aluminum scrap could be recycled through chloride molten salt electrorefining to remove magnesium prior to casting. Magnesium will in all likelihood be used more extensively as corrosion problems are solved and as new alloys are developed. Electrorefining offers the opportunity to allow total recycling of the materials. Electrochemical machining may become the shaping method of choice with the adoption of materials difficult to machine in any other way, such as composites and powder metallurgy structures. Electro-Organic Synthesis If electro-organic products are classed as commodities or specialty chemicals with an extreme price range of about 5 to 1, a first approximation estimate may be made as to the price that can be paid for the reactor expressed as dollars of capital investment per square meter of electrode surface. At a constant current density of 1000 A/m2, there is an order of magnitude higher investment allowed in reactors that can yield a good return for high-priced specialty chemicals over commodity chemicals commanding one-fifth the price. Since three-dimensional electrode reactor designs such as bipolar plates, trickle beds, and monopolar or bipolar packed or fluid beds cost only $400 to $1000 per square meter of active electrode surface, as compared to $2500 to $10,000 for more conventional planar designs, only the former can be used profitably in the production of low-cost commodity chemicals.
77 Further research in novel high-surface-area three-dimensional bipolar systems, therefore, could advance the industry into low-cost chemicals by greatly reducing capital intensity through improved space-time yield. In the past, many candidate processes failed to yield the required return on investment because reactor designs at the allowable capital investment were not available. In spite of this, applications for electrosynthesis with the required profitability are growing as electrochemical engineering with a "systems" approach develops. To apply some of the newer chemical proposals it will be necessary to develop even more accurate mathematical design models and more innovative three-dimensional low-voltage-drop electrodes. Research Needs For implementation of the ideas suggested under the heading "Emerging Technologies," it will be necessary to carry out research focused on the following fundamental topics: · Study the fundamental factors at interfaces that control charge-transfer reactions, in order to develop new electrocatalysts Develop new electrochemical reactor designs that permit very high current densities by moving the electrolyte at high velocity between very close electrodes, taking advantage of the third dimension through bipolar plates and designing means for separating the anode and cathode products Develop new electrode materials and the modification of electrode surfaces by investigation of conductive polymers, organometallic conductors and semiconductors, and the phenomena of absorption and covalent attachment · Develop sensors using electrochemistry as well as new electroanalytical methods · Study electrocrystallization phenomena and electrode surface morphology when deposition and stripping are taking place · Determine kinetics and mechanisms of electrode reactions shown to be potentially useful (i.e., carbon monoxide and dioxide reduction, alkali metal deposition, solution redox reactions, and oxygen reduction) to permit the design of highly efficient electrolytic cells (55) · Investigate possible new ionizing solvent media suitable for reactive metals and for electro-organic synthesis
78 MEMBRANES Membrane processes find use in many technological fields and are also essential to the organization and dynamic behavior of living matter via cells, nerves, and muscles (56-63~. Although many technological applications lie outside the traditional electrolytic area, some of the critical barriers to advancing membrane science involve electrochemical phenomena. The applications are varied and currently include health care (kidney dialysis), chemical processing (desalting by electro- dialysis and reverse osmosis, separation of gases), energy conversion and storage (separators for batteries and fuel cells), food and biochemical processing (desalting and demineralization of food products, controlled release of drugs), environmental operations (treatment of spent pulping liquors and electroplating wastes), sensors (ion-selective electrodes), and electrochemical synthesis (chlor-alkali ion exchange membranes). The membrane process for chlor-alkali production is a major success story in membrane technology. It led to substantial energy savings and to replacement of asbestos- and mercury-based technologies. Development was spurred by Japanese environmental legislation; no economic advantage was apparent in the early stages of the development process. However, this new technology has subsequently been shown to save up to one-third of electrical costs compared to the less-efficient processes. Emerging applications for membrane processes are based on the fact that membranes are potentially very energy-efficient for difficult separations. The inherent energy advantage arises because membrane separations require no phase changes and thus avoid energy-consuming latent heats associated with such phase changes. Applications of present and growing importance include biotechnology (large-scale protein separations), natural and strategic resource recovery (extraction of metals from low-grade ores), chemical processing of coal (water cleanup and recycling), environmental processes (river desalting, toxic metal recovery), health care (artificial organs, skin sterile filtration, targeted drug delivery), microelectronics (ultrapure water and gases), sensors (drug dosage monitoring, immunological probes, and agricultural applications), electric vehicles (solid polymer electrolyte fuel cells), and electrochemical technology (especially electro-organic synthesis). Membrane processes are modular in their nature, so expansion and/or replacement can be spread out over time, based on market needs and capital availability. The needed performance capabilities required of membranes vary with the nature of the application and generally involve the need for high permeability, selectivity, low cost, slow degradation, wide temperature operating range, and adequate mechanical strength. Membrane processes are successful only when the associated engineering
79 problems are dealt with effectively. In general, membrane engineering methods require development beyond their present ad hoc stage. There are a number of barriers to achieving the improved performance needed for these emerging applications. These include development of improved membrane materials, improved support structure geometries, better fabrication techniques, process improvements, and innovation of new membrane concepts. New functional polymers need to be developed that may be fabricated, modified, and optimized for specific appli- cations. Such membranes must be able to operate under a wide range of conditions (temperature, solvents, and oxidizing-reducing conditions), resist fouling, and exhibit compatibility with biological systems. Improved methods are needed for achieving thinner membranes to permit higher fluxes, for supporting them with mechanically robust structures to avoid fouling, and for achieving compatibility with systems that contact the membrane. The integration of membrane operations with chemical process flowsheets must be made more facile in order to permit identification of attractive process candidates. Improved characterization of chemical and physical properties is therefore important. Such properties include conductivity, water-vapor pressure, freezing-point depression, gas solubilities, and permeabilities. The development of membranes that contain active elements such as catalysts, affinity reagents, or immobilized enzymes will lead to membrane reactors that perform catalysis, separation, and concentration in a single device and in a relatively energy-efficient manner. Biomembranes are nature's supreme molecular organizers, and the development of membrane processes that mimic their behavior would have far-reaching implications in health care, chemical synthesis, and solar energy conversion. Along more general lines, there are several areas where fundamental scientific problems need to be addressed. These include ~ Molecular aspects of transport in membranes so that models having predictive capability can be built, which would lead to the design of membranes at the molecular level in order to impart specific permselectivities · Diffusion of condensable gases through nonequilibrium systems such as glasses, and transport of macromolecular permeants such as those encountered in separations and biomaterials · Reaction and transport at surface-catalyzed membranes used in solid polymer electrolytic cells and energy-conversion devices · Structure-permeability relationships to clarify how diffusion anomalies are related to morphology and ionic hydration
80 · Tailoring polymers for specific membrane separations and for specific mobile ions · Ion fractionation with specific morphologies, and ion exchange selectivity dependence on morphology ~ Invention of several types of membranes those for mediating redox catalyst systems for chemical synthesis, chemically stable anion exchange membranes, and polymer membranes with gradually changing structural properties along the direction of transport · Greater membrane chemical stability in strongly oxidizing and reducing environments, over wide ranges of pH, and in nonaqueous solvents Membrane research and development requires contributions from several disciplines, including synthetic polymer chemistry, polymer physics, electrochemistry, and chemical engineering. It has often been difficult to assemble the critical mass of these disciplines in a program of membrane development. To date, people working in the membrane field have taken whatever polymers have been available. Development costs are often too high for a specific application or single user. At present there is neither the scientific knowledge to design membrane polymers on a rational basis nor the requisite multidisciplinary interactions for tailoring membranes for particular applications. Cooperative efforts are needed to identify opportunities for new membrane-based process alternatives and to create teams that bridge disciplines in order to conduct joint development projects. Impediments to university-industry interaction may arise, however, since academics need access to membrane fabrication techniques whereas the industry, for proprietary reasons, is reluctant to part with such knowledge. The following research areas hold promise for advancing major technological growth: · Stable anion exchange membranes and low-cost cation exchange membranes · Basic transport studies at the molecular level aimed at molecular design of membranes · Invention of new membranes that contain active elements · Biocompatibility of membranes with living systems to permit affinity-discrimination, sensing, and molecular recognition
81 MICROELECTRONICS Electrochemical processes are an essential element in the menu; facture of modern electronic and photonic systems. The quality and reliability of these systems are controlled by bulk and interracial ionic charge transport processes (64-71~. Looking toward the 1990s, one sees rapid evolution in information gathering, transmitting, storing, and processing systems. These will be increasingly parallel and will thus more closely resemble biological networks. Just as in biological systems, the elements can involve electrochemical mechanisms of charge transport. The electrochemical coupling of microelectronics and the central nervous system, already a subject of intensive (and in certain instances successful) experimenta- tion, will surely expand in the near future. Processes for the Manufacture of Microcircuits The chemical, electrochemical, and photoelectrochemical etching processes by which microelectronic components are made are controlled by electrochemical potentials of surfaces in contact with electrolytes. They are therefore dependent on the specific crystal face exposed to the solution, on the doping levels, on the solution's redox potential, on the specific interracial chemistry, on ion adsorption, and on transport to and from the interface. Better understanding of these processes will make it possible to manufacture more precisely defined microelectronic devices. It is important to realize that in dry (plasma) processes many of the controlling elements are identical to those in wet processes. By using light, it is possible to create an excess of electrons or holes locally in a doped semiconductor and thereby increase or decrease the rate of etching in either dry or wet processes. Structures that cannot be produced by any other means, such as narrow holes with extreme aspect ratio, have been produced by photoelectrochemical etching. Because not all of the complex interrelated heat, mass, and electron transport processes involved are as yet understood, the results are not always predictable. Metals such as gold are photoelectroplated onto semiconductors to form micropatterns. Such plating saves steps in lithography and masking. Photoelectrochemistry also has a central role in photolithog- raphy and in electron-beam lithography with inorganic resists, based on Ag2Se films on GeSex. The unique properties of these resists, such as their superior resolution, tolerance to defocusing, and tolerance to overexposure or underexposure, derive from the super- linearity of their photoresponse, which derives, in turn, from fast ion transport in a 100 A thick 9-Ag2Se film and from phototransport
82 of silver ions from the Ag Se solid electrolyte into the GeSe glass. Multilayer semiconductor structures of modulated chemical composition have also been made electrochemically. Areas of fundamental electrochemistry that are particularly relevant to the manufacture of microelectronic components include the sciences of semiconductor electrochemistry, ion transport, corrosion, plating, microcelis (of ~ to 20~m dimension), photoetching, and photoelectro- plating. Processes for the Manufacture of Interconnecting Networks Between Microelectronic Components Electrochemistry is a central theme in the interconnection of chips and other microelectronic components. The manufacture of printed wiring boards, such as single-layer, multilayer, or flexible boards, involves electroplating of the conductor that forms the electrical paths. The corrosion of these paths and the interfacial stability of the conductor- polymer composites that determine the reliability of these inter- connections are electrochemical problems. Electrochemistry also has a central role in the manufacture and reliability of hybrid integrated circuit packages with ceramic substrates; in single-layer and multilayer ceramic boards for chip mounting, and in advanced multilayer silicon-substrate-based inter- connecting networks. Because the trend in information processing and storage is toward increasingly parallel networks, where a large number of chips communicate with each other at high rates, structures with increasingly complex three-dimensional interconnecting structures are now being built. The science of electrochemistry is central to their manufacture, performance, and reliability. The relevant fundamental areas of electrochemistry are ion transport in microcells; modeling of microcells of high aspect ratio; electrochemical leveling; electro- chemical stability of metal-polymer interfaces; and humidity- and temperature-dependent transport of ions through polymers, ceramics, and glasses and at their interfaces with metals and with the atmosphere. Processes for the Manufacture of Lightwave Communication Devices Lightwave communication devices, such as integrated microlenses that focus light from light-emitting diodes onto the ends of optical fibers, are manufactured by photoelectrochemical etching of III-V semi- conductors. Other components such as microgratings are also made by photoetching. Electroplating is also relevant to lightwave communi- cation devices in the formation of electrical contacts, both ohmic and Schottky.
83 The particularly relevant areas of fundamental electrochemistry are semiconductor electrochemistry, photoelectrochemistry, and transport in microcells. Reliability of Microcircuits Producers know only too well that sodium and other ions will wreak havoc with integrated circuits. Their presence causes electrochemical corrosion of the very fine metal runners. Also, the transport of ions and their redistribution perturbs the electrical operating characteristics of MOS (metal-oxide-semiconductor) and other integrated circuits. The cause of these effects is in the spacing of the metal runners which is 1 to 2,um in today's circuits, and will be of 0.5 to 1,um within a decade. Because of the small distances, the electric fields are high and the transport of ions on the surfaces of the microcircuits, when ions are present, is rapid. The electrolytic processes corrode the metal runners and lead to accumulation of certain anions and cations on different regions of the surface. Because some ions are more strongly adsorbed than others, their transport introduces local electric fields that perturb the operation of microcircuits. The metal runners corrode either directly or indirectly. In direct corrosion, the metal, usually aluminum, is electrolytically oxidized to compounds of A13+. In indirect corrosion, electrolysis causes a local change in pH. Aluminum is attacked both at excessively high and at excessively low pH. In silicon devices the surface in which ions are transported consists of amorphous hydrated SiO2. Although it is this layer that determines the ion transport and therefore the reliability characteristics of microcircuits, virtually nothing is known about the properties of the layer as a solid electrolyte for example, the variation of its composition with temperature and humidity, or the solubility of electrolytes, the temperature-dependence of their solubilities, and the diffusivities or mobilities of ions in these films. Furthermore, the nature of the anodic and cathodic electrode reactions in the surface electrolytic processes, for either conventional aluminum metallizations or for newer metallizations involving refractory metals (Mo, W. Ta, Ti) and their silicides, has not been determined. Also unavailable are data on what ions are selectively trapped in the hydrated SiO2 surface, although it is this trapping that perturbs the operating characteristics of microcircuits. To avoid surface corrosion processes, the microelectronic industry takes three precautions: it avoids the use of solvents and reagents that may leave an ionic residue on the microcircuits; it encapsulates the integrated circuits; and it packages the microcircuits in plastic containers. When used in weapons, in space, or in undersea communi
84 cation systems, the circuits are packaged in particularly expensive ceramic hermetically sealed packages. Packaging and encapsulation now constitutes 15 to 50 percent of the cost of microcircuits. If one adds the expense of careful exclusion of ions in the processing steps (use of deionized water, high-purity solvents, sodium-free reagents, etc.), the cost of this ignorance of surface electrolytic processes in micro- circuits is even higher. Encapsulants of integrated circuits were originally introduced to prevent mechanical damage and to slow down corrosion by reducing transport of oxygen and water to the corroding metals. Today it is recognized that encapsulants reduce corrosion by reacting with regions on the hydrated SiO2 surface, thus slowing the lateral transport of ions. Some encapsulants also act as ion traps. It is reasonable to expect that, if methods for quantitative measure- ment of the transport of ions in surface phases of semiconductors are developed, the way will open to the exploration of chemical and physical modification of these surface phases. The goal is to make these less conductive solid electrolytes-i.e., surface phases in which ion transport is reduced. Such modification is likely to reduce the cost of encap- sulation and packaging and increase the reliability of microcircuits. Reliability of Interconnecting Networks Multilevel interconnecting networks consist of layers of metal runners isolated from each other by a dielectric. At defined points, runners in different planes are electrically contacted by metal columns. The purpose of these three-dimensional networks is to carry electrical signals at high speed. Therefore, the resistance and capacitance of the interconnecting networks must be low. Low resistance in a dense network of conductors implies that the runners must be made of highly conductive metals such as copper. Low capacitance implies that the layer of the dielectric must be thick and that its dielectric constant must be low. Usually, the layers isolating the metal layers are polymers like polyimides. Because oxygen diffuses to the polymer- copper interface, the copper oxidizes. If complexing functions like carboxylic acids are formed upon oxidation of the polymer or are intrinsically present, they complex the copper cations, causing both gradual dissolution of the metal and a change in the electrical properties of the dielectric. Because multilayer interconnecting networks are an important element of advanced chips and parallel processors, it is essential that an understanding of the corrosion processes that affect their reliability be developed. Needed are methods to quantify metal corrosion and ion transport in polymers and means to identify electrochemically reliable metal-polymer systems.
85 Electrochemistry of Highly Parallel Processors The production of future generations of highly parallel processors requires manufacturing processes of unprecedented stringency in yield and precision. These processors will have dimensions of 10 to 100 cm2 and will consist of approximately 104 VLSI chips, with each chip connected to every other chip by approximately 102 metal runners, accommodated in a three-dimensional network. Their design requires. as · , .~ e , · · ~ seen In the previous section, ~n-depth understanding of the interracial electrochemistry between metals and dielectrics and of ion transport in channels of diminishing size that connect metal runners in different planes. Formation of the networks requires extreme control over the plating process so that all columns have precisely identical lengths and perfectly flat tops; nonidentical lengths or curved tops lead to defects in the three-dimensional structure. The most relevant areas of fundamental electrochemistry are modeling of microcells and interracial corrosion. Electrochemistry of Content-Addressable Memories Beyonc} the evolution of von Neumann computers lies the beginning of the science and technology of content-addressable memories now being experienced. These approach more closely the way the human mind works. They are more fault-tolerant and associative; i.e., they function with imprecisely defined information and with imperfect circuit elements and can relate information elements to each other. State-of-the-art associative memories are based on variable-resistance network "opens" and variable degrees of "shorts." The variable shorts can be generated electrochemically both in polymers and in inorganic materials e.g., by the reductive electrochemical diffusion of Na+ into WO3 films, which produces conductive tungsten bronzes, or by the oxidative diffusion of C1O4- into polyalkyl thiophene films, which produces a conductive polymer. Such circuit elements have already been made. The most relevant areas of fundamental electrochemistry are solid-state electrochemistry and the modeling of microcells. Electrochemistry of Nerve-Electronics Interfaces The electrochemistry of nerves has been the subject of several decades of study. Ion transport across cell walls is a key element in the functioning of nerve cells, and a network of nerves can be viewed as a set of electrochemical, membrane-containing microcells that are coupled by chemical messengers. Interfaces between nerves and
86 microelectronic triggers that are crude by biological standards have already been implemented and are in limited use in rehabilitation. The modeling of coupled electrochemical microcelis, progress in capacitive biocompatible microelectrodes, and the creation of precisely tailored arrays of microelectrodes are particularly relevant to the coupling of microelectronics and nerves. SENSORS Electrochemical sensors have demonstrated their potential to provide sensitive, selective, reliable, robust, and inexpensive means for solving otherwise intractable problems of chemical analysis (72~. They have proved to be well suited for application to both gas phase and liquid phase problems, including clinical chemistry and research in the life sciences (73,74~. Some noteworthy devices include miniature sensors for real-time monitoring of oxygen partial pressure in high- temperature automobile exhausts, lightweight portable monitors for a variety of toxic gaseous species (e.g., carbon monoxide, nitric oxide, nitrogen dioxide, hydrogen sulfide), ion-selective electrodes for measurement of electrolytes in clinical applications (sodium, potassium, calcium, etc.), ultramicroelectrodes for in viva determination of glucose and of biologically active species, detectors for liquid chromatography of drugs used for neurological disorders and for therapeutic drug monitoring, and potentiometric sensors for quantification of low concentrations of electroactive species. With the exception of potentiometric sensors, no consistent pattern of federal support has existed. Recent advances in microelectronic fabrication techniques, in development of modified electrode surfaces and ion-selective membranes, and in availability of new materials give promise for development of new electrochemical sensors. For both gas and liquid sensors, the possibility of much higher sensitivity exists. Lower detection limits are possible for environmental, clinical, and general analysis situations. Sensors developed to date are primarily based on classical and relatively unsophisticated approaches. With newer methodologies and device designs, one may anticipate at least a ten-fold improvement in detection limits. Among the methods that have considerable promise but that are yet to be significantly exploited are pulse electrochemical techniques, impedance methods, flow-injection analysis, the use of nonaqueous solvents in the sensor, the combined use of chemometrics and multi- electrode measurements for analysis of complex mixtures, the use of ultramicroelectrodes in applications outside the clinical and biological areas, and rapid deaeration of flow systems.
87 Electrochemical sensors are based on selective interracial charge generation and localized charge transport. Within the past decade, major advances have been made in recognizing basic principles that unite the wide variety of systems encountered in practice. From these principles and the working out of charge, potential, and composition Profiles. Prediction of the properties of materials for the design and , , ~ . . · ~ . ~ ~ ~ ~ . ~ ~ ~ ~. construction of new sensors has proceeded at an increasingly rapla rate. Key scientific challenges include the design of new molecules and substrates that possess the high transport selectivity required for new and improved sensors. The discovery and molecular characterization of new sensing elements will include surfaces modified with specific electrocatalysts and/or enzymes, ion-specific membranes, fast ion- conducting ceramics and glasses, conducting polymers, and semiconductor materials. The use of surface analytical techniques to probe the molecular details of the sensing mechanism of these materials will contribute to improved sensitivity i.e., reduced interference by other species. Closely related is the problem of sensor design for use in very low concentrations of species. Theoretical characterization of transport of sensed materials to and from the sensor interface must advance significantly to design reliable and reproducible sensors and to predict their responses in the transient and steady states. The invention of new devices would be aided significantly by transposing the principles of potential- and current-generating sensors to related field-effect devices, by capitalizing on improved knowledge of permselectivity in polymer films, and by exploring more deeply the principles of charge cancellation reactions for immunological applications. Invention of new manufacturing methods based on the microelectronics industry, coupled with new sensing materials and methods of detection, would represent a significant advance. For example, new sensors based on redundant arrays of microsensing devices may be key to low-cost reliability, which is essential to many applications. A significant barrier to developing improved sensors is the lack of focus for support of fundamental studies and the inadequate marshalling of multidisciplinary skills for development efforts. Much sensor development now occurs in connection with health science needs, defense needs, or the requirements of other mission-oriented agencies. Without a focus of support, it is currently difficult to undertake fundamental, systematic studies that would explore a new generation of sensing techniques and materials. Sensor technology is multidisciplinary, both in the assembly and characterization of the sensing element and in the fitting of that element into the specific system in the field. Manufacturers of instruments often do not have specialist teams with adequate breadth to develop novel techniques into commercial devices. As a consequence, there are missed opportunities in the conception of
88 new methods as well as poor transfer to the marketplace of those concepts that do arise. In general there appear to be no generic problems that are inherent to the development and fabrication of vastly improved electrochemical sensors. The environment in which a sensor operates may generate materials problems (such as in blood or at high temperature or pressure), but these are not appreciably different from those existing for other instruments and devices exposed to the same environment. It is unlikely that more sophisticated sensors would give rise to intract- able materials or manufacturing problems. The present role of the federal government in support of sensor science and technology is unfocused. There is no clearly evident federal funding agency where a fundamental sensor proposal might attract funding without being directly linked to a specific mission-oriented problem. Improved federal sponsorship of fundamental investigations aimed at developing principles of advanced sensors would play a major role in promoting technological progress. The commercialization of new and improved sensors by U.S. manufacturing firms represents a very significant and strategic economic benefit. Research areas that hold high promise for advancing technological growth include ~ Enhancing sensor selectivity by discovery and molecular characterization of new and improved sensing elements · Invention of new fabrication methods, based on microdevice technology, to improve reliability, reproducibility, and cost REFERENCES 1. Committee on Battery Materials Technology. Assessment of Research Needs for Advanced Battery Systems. National Materials Advisory Board, NMAB-390. Washington, D.C.: National Academy Press, 1982. Committee on Fuel Cell Materials Technology in Vehicular Propulsion. Fuel Cell Materials Technology in Vehicular Propulsion. National Materials Advisory Board, NMAB-411. Washington, D.C.: National Academy Press, 1983 . Extended Abstracts: Seventh Battery and Electrochemical Contractors' Conference. U.S. Department of Energy, CONF-851146-Absts, Nov. 1985.
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